Currently, the optical elements have been widely applied, and therefore the demand of the antireflection films becomes greater and greater in order to decrease the reflectance and to increase the transmittance of light after passing through the optical elements. Several methods for forming the antireflection films are available, including physical and chemical vapor deposition methods. Currently, most optical coatings are done by the physical vapor deposition. The vapor deposition is a coating technique, and is operated in a vacuum condition by heating the coating material source up to its melting or evaporating temperature, so the coating material is evaporated and then deposited onto the surface of the substrate. During the deposition process, the substrate is usually heated, so the evaporated atoms or molecules have enough energy to move on the surface of the substrate, and therefore the films can be evenly formed. Consequently, the temperature of the substrate has an important influence on the properties of the deposited films. Generally, if the glass substrate is used, the substrate is heated up to 300° C. so as to form good bonding between the films and the substrate to avoid peeling.
The conventional antireflection films are usually done by depositing at least two film layers with different refractive indexes on a transparent substrate. Usually more than three layers are superposed on the substrate, and therefore the optical interferences among the multiple layers can be applied to reduce the reflected light. Although the vapor deposition has the advantages of the high deposition rate and the convenience to form the multiple layers, however the substrate material unable to resist the high heat is not suitable for use in this method due to the concern of the high operation temperature during processes. For example, conventionally the coated films on the plastic substrate mostly contain one or two layers due to the low resistance to high heat for the plastic substrate. Accordingly the antireflection effect is limited owing to the too few coated layers. If more layers are coated on the plastic substrate, the phenomena of the poor adherence or even peeling and cracking occur.
In order to eliminate the drawbacks of the conventional techniques, the new concepts and the solutions are proposed in the present invention so as to solve the above-mentioned problems. The present invention is described below.